1. Field of the Invention
This invention relates to a method and apparatus for electrical well logging and, in particular, to a method and apparatus designed to simultaneously measure, in a borehole, the electrical resistivities of the subsurface earth formations extending over relatively small and relatively large lateral distances from the wall of the borehole. Even more particularly, the invention relates to a method and apparatus utilizing linear components in a subsurface electronic circuit for reducing the dynamic range of the voltages and currents which are measured to derive ratios indicative of the apparent resistivity of the earth formations surrounding the borehole.
2. Description of The Prior Art
It is well known in the art of electrical well logging to use electrical fields produced and adjusted to focus the current emitted by the electrode in the borehole, forcing the current to follow a path confined to a zone substantially perpendicular to the wall of the borehole. Accordingly, the early logs were obtained by recording the variations and the differences in potential, appropriately chosen and related to the flow of current through subsurface earth formations, and deriving the electrical resistivities of the formations extending over relatively large lateral distances from the wall and borehole. Other types focused the emitted current over shorter lateral distances so that the logs derived from the current flow measured represented, with greater exactitude, the electrical resistivities in the neighborhood of the wall. The influences of the liquid filling the borehole, mudcake and material located at a great distance from the wall of the borehole being practically eliminated thereby.
One prior art apparatus, described in U.S. Pat. No. 2,712,630 issued on July 5, 1955 to Doll, is directed to the use of a system of electrodes whereby controlled electric fields emitted into the borehole, in a path perpendicular to the wall thereof, may be focused such that the earth formations at different lateral depths from the wall of the borehole are measured.
In trying to provide near simultaneous determination of the resistivities at various lateral depths, another prior art apparatus, described in U.S. Pat. No. 3,660,775, issued on May 2, 1972 to Janssen, utilizes a similar set of electrodes, connected to permit switching electrode focusing of the current to measure resistivities at medium or far lateral distances from the borehole simultaneously. The apparatus combines the measurements to provide a determination of the apparent resistivity of the earth formation. Commutating the signals between sets of electrodes during the traverse of the borehole by the logging instrument permits nearly simultaneous medium and far distance determinations of the resistivities at a particular lateral portion of the borehole.
More recently, the prior art has again been directed to circuitry utilizing a pair of frequencies, suitably connected to provide for the simultaneous measurement of the near and far resistivities of the earth formations. Notably, U.S. Pat. No. 3,772,589 issued to Scholberg on Nov. 13, 1973 discloses apparatus of the type similar to that used by Doll.
Additionally, Scholberg recognized that in resistivity well logging applications, as with many measuring systems, it is required to measure the value of a parameter to within some specified degree of accuracy rather than just detecting a change in that parameter. The precision to which the unknown is to be resolved and the range of the values of the unknown determine the dynamic range requirements of the measuring system.
By reducing the dynamic range of the measured parameters, increased measurement accuracy can be achieved over smaller measurement ranges.
The above mentioned Scholberg apparatus permits the wide dynamic ranges encountered in measuring formation resistivities to be reduced by maintaining constant power for impressing the measuring currents and voltages across the formation. However, the system must of necessity rely on the use of non-linear circuit elements such as shown in the surface electronics portion of the Scholberg patent.
The disadvantages of the prior art are overcome and novel methods and apparatus are provided for simultaneously determining the apparatus resistivities of each formations surrounding a borehole at near and far distances utilizing linear circuit elements interconnected in a subsurface electronic circuit which reduces the dynamic measurement range of the signals within the surrounding earth formations.